1. Field of the Invention
The present invention relates to an encoder for detecting the position and/or speed of an electric motor and, more particularly, to a self-diagnosing system for an encoder, which detects by a self-diagnosis the occurrence of a malfunction or abnormal state during operation of the electric motor.
2. Description of the Related Art
As is generally known, an encoder is widely used to detect, e.g., the position and/or speed of a servomotor that drives an axis (i.e., a control axis) of a machine tool or an industrial robot. In this configuration, a malfunction or abnormal state often occurs in the encoder itself or the electric motor due to various internal or external factors. Therefore, it has been conducted that the encoder performs a self-diagnosis on whether a malfunction or abnormal state has occurred in connection with a predetermined check item, and that, when the result of the self-diagnosis is obtained as to indicate the occurrence of a certain malfunction or abnormal state, the encoder transmits or transfers the result of the self-diagnosis to a control unit of the axis as alarm information. In this case, the alarm information transferred to the control unit is used for a maintenance service, such as a root cause analysis of the occurrence of the abnormal state.
However, the data (i.e., the alarm information) transferred from the encoder to the control unit is often insufficient to be used to elucidate the cause of the abnormal state occurring in the encoder or the electric motor, because of a limit in the data transferring capacity. In other words, the output from the encoder, as the alarm information transferred to the control unit when the abnormal state occurs, is generally only one self-diagnosis result concerning a check item showing the abnormal state (which means that one abnormal-state judgment factor shows the abnormal state), or the logical sum of a plurality of diagnosis results concerning the check item (which means that at least one abnormal-state judgment factor shows the abnormal state). Therefore, it is generally difficult to elucidate satisfactorily the cause of the abnormal-state occurrence, by using only the alarm information displayed or stored in the control unit.
In the case where an encoder falling into an abnormal state is replaced with another one and, at a later date, a specialty division investigates the cause of the abnormal state occurring in this encoder, the encoder sometimes does not manifest the phenomenon of the abnormal state when the encoder is operated again. In this case, the elucidation or the cause of the abnormal-state occurrence becomes more difficult.
It has been conventionally known that a storage section for storing information required to control an electric motor is incorporated into the encoder. For example, Japanese Utility Model Publication (Kokai) No. 1-143297 (JP-U-01-143297) discloses a servomotor with encoder, including a storage section for storing inherent information corresponding to the type of a servomotor. The storage section provided in this servomotor with encoder is used to store inherent information, such as identification (ID) information of the servomotor (such as specification data, date of manufacture, the serial number of the unit motor, etc.), and detection-precision correction data of the encoder. However, the storage section does not store internal information on the encoder (i.e., data indicating an abnormal state) at the instant a malfunction or abnormal state occurs in the encoder or the servomotor.
On the other hand, Japanese Patent Publication No. 3183427 (JP-B-3183427) discloses an abnormal-state detecting system, in which a storage section for storing data (i.e., alarm information) indicating an abnormal state at the instant an abnormal state occurs in the electric motor is incorporated into an encoder. This abnormal-state detecting system is not a self-diagnosing system, but one in which a driver (i.e., a control unit) detects an abnormal-state in the electric motor. When an abnormal-state is detected, the driver interrupts the power supply to the electric motor, and transmits alarm information indicating the motor malfunction to the encoder. Then, the storage section provided in the encoder stores the alarm information transmitted from the driver. The alarm information stored in the storage section can be read out through the driver, or through a suitable reading unit in a case where the driver is omitted.